1. Field of the Invention
The present invention relates to a vehicle-mounted electric rotating machine for a vehicle such as a passenger car or a truck.
2. Description of Related Art
A vehicle-mounted generator is for supplying charging power to a battery of a vehicle through a charging cable connected to the output terminals thereof, the battery supplying operating power to various electrical loads mounted on the vehicle. If the charging cable comes off or comes loose from the output terminals of the generator or the battery terminals when the generator is generating power, there occurs an instantaneous high voltage called “load dump” at the output terminals of the generator. The high voltage can exceed 100 V depending on the output current of the generator. This high voltage may break or damage the electrical loads mounted on the vehicle or components of the generator. Accordingly, it is necessary to take countermeasures against the load dump. There is known a vehicle-mounted generator with a protection circuit against load dump. For example, refer to Japanese Patent Application Laid-open No. H09-219938. The vehicle-mounted generator described in this literature has the structure in which the low side elements of a bridge circuit of the generator are constituted of MOS transistors, and these MOS transistors are turned on when a load dump occurs and the output voltage of the generator exceeds a reference voltage. When the output voltage falls below the reference voltage after the turning on of the low side elements (MOS transistors), the MOS transistors are turned off so that the bridge circuit can resume its rectifying operation.
However, the vehicle-mounted generator as described in the above literature has a problem in that, since the operating voltage of the protection circuit is supplied from a battery charged by the generator, if the charging cable comes off the terminals of the battery causing a load dump, the protection circuit may become unstable. It is possible to generate the operating voltage of the protection circuit from the output voltage of the vehicle-mounted generator to cope with this problem. However, if the output voltage of the vehicle-mounted generator drops excessively for a long time as a result of the operation against the load dump, it becomes difficult for the protection circuit to operate normally. Since the vehicle-mounted generator has to repeat temporary power generating operation to prevent the output voltage from dropping more than necessary, it takes a long time for the output voltage to return to its normal value because it takes a long time for the energy accumulated in the stator winding due to the load dump to disappear.